.\"	$NetBSD: __CONCAT.3,v 1.8 2013/10/17 20:43:49 wiz Exp $ $
.\"
.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jukka Ruohonen.
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.Dd October 17, 2013
.Dt __CONCAT 3
.Os
.Sh NAME
.Nm __CONCAT ,
.Nm __STRING
.Nd argument substitution
.Sh SYNOPSIS
.In sys/cdefs.h
.Ft xy
.Fn __CONCAT "x" "y"
.Ft const char *
.Fn __STRING "x"
.Sh DESCRIPTION
The
.Nm
macro makes use of the
.Xr cpp 1
preprocessor to concatenate two tokens.
When the macro is expanded,
.Fa x
and
.Fa y
are combined into a single token, provided that the result forms a valid token;
two tokens that together do not form a valid token can not be concatenated.
This is known as
.Dq token concatenation
or
.Dq token pasting .
.Pp
The
.Fn __STRING
macro uses the conventional
.Sq #
preprocessing operator to replace the argument
.Fa x
with a string literal.
This is also known as
.Dq stringification .
.Sh EXAMPLES
The following two
.Xr printf 3
calls produce the same output:
.Bd -literal -offset indent
#define Net	0x01
#define	BSD	0x02

#define NetBSD	"NetBSD"

(void)printf("%s\en", __CONCAT(Net, BSD));
(void)printf("%s%s\en", __STRING(Net), __STRING(BSD));
.Ed
.Sh SEE ALSO
.Xr cpp 1 ,
.Xr cdefs 3
.Sh HISTORY
The
.Fn __CONCAT
and
.Fn __STRING
macros first appeared in
.Nx 1.3 .
.Sh CAVEATS
Many small details direct the proper use of the macros.
For example, while all leading and trailing whitespace is ignored when
.Fn __STRING
is used, it is undefined whether
.Xr cpp 1
puts white space between the tokens when
.Fn __CONCAT
is used.
It can be also noted that the C preprocessor converts all
comments to whitespace before any macros are even considered.
The use of either macro is discouraged in complex constructs.
.Pp
Use of this macro is non-portable; this is part of the implementation
namespace and should only be used in
.Nx
code.